One of the caveats in the legislation will limit the amount of gambling losses that can be used to offset winnings to 90%. That’s down from the 100% gamblers can currently claim.

That’s unlikely to hit casual gamblers too hard, but frequent Vegas (and other casino town) visitors and professional gamblers could see much higher tax bills starting next year.

Casinos are required to issue tax paperwork when a player wins $1,200 or more. But gamblers are required to keep up with their losses themselves—and the IRS often asks them to back up those claims, something that’s not always easy.

Casinos always have the advantage over gamblers, so even those who rack up big wins generally have big losses as well. Traditionally, those losses have been used to lower the tax bill at the end of the year. Under the new rules, though, they’re likely to pay a higher amount or could pay significant taxes even if they just break even for the year.

Nevada Congresswoman Dina Titus, who highlighted the clause on social media, has introduced legislation that will act as “a legislative fix that fairly treats gaming losses in the tax code”—but as a Democrat, she might have a tough time pushing it through.

Professional gamblers have decried the changes, as well.

“Let’s say that over the course of all the sessions that we played throughout the year, we won $5.2 million and we lost $5 million dollars for a net of $200,000,” said Phil Galfond on social media. “Now, we would pay as if we won $5.2 million, minus 90 percent of $5 million, which is $4.5 million for a fake net of $700,000… So you would make $200,000 during the year and pay tax as if you made $700,000.”